Considerable evidence indicates that the 24 hr. fluctuations observed in behavioral, physiological and cognitive functions depend upon endogenous rhythmic mechanisms. In mammals, the generation of these 24 hr. or circadian rhythms is controlled by neural systems. Previous work indicates that the mammalian suprachiasmatic nucleus (SCN) is a very important component of the neural mechanism responsible for circadian rhythms. Therefore, a better understanding of the pathways connecting the SCN to the rest of the brain should increase our knowledge about the circadian regulation of behavior. The objective of the proposed research is to investigate the functional anatomy. The objective of the proposed research is to investigate the functional anatomy of the efferent connections of the SCN. To achieve this goal, neuroanatomical studies using anterograde and retrograde tract-tracing methods will be used to determine the connections of the SCN. In addition, mutant strains of rats and hamsters will be studied to determine if neuroanatomical anomalies in the SCN are correlated with deficits in the expression of circadian rhythms in behavior. Behavioral rhythms including the sleep cycle will be monitored under constant environmental conditions. Seasonal cycles in reproduction are also regulated by the SCN and by the connections between this nucleus and other hypothalamic sites. Experiments will be conducted to establish the functional anatomy of the hypothalamic circuits involved in the regulation of seasonal cycles of reproductive and regulatory functions.